The present invention relates to a power source for an electric motor that drives a re-circulation pump for re-circulating the reactor coolant in a boiling water nuclear reactor, and to a method of controlling the power source. More particularly, the invention relates to a variable-voltage variable-frequency power source and a method of controlling the same.
Typically, the output power of a boiling water reactor is controlled by changing the flow rate of the reactor coolant re-circulated in the reactor pressure vessel through the reactor-coolant re-circulation system. The reactor-coolant re-circulation system includes re-circulation pumps, electric power sources for the re-circulation pumps, a re-circulation flow controller, and the like.
Loop pipes connect the re-circulation pumps to the reactor pressure vessel. The re-circulation flow controller generates a speed demand signal. This signal increases or decreases the output of the power source for the re-circulation pump, thus controlling the electric motor. The speed of the re-circulation pump is thereby controlled. This changes the coolant flow rate in the reactor pressure vessel to thereby control the output power of the reactor.
A method of controlling the rotational speed of the re-circulation pump is known, in which a variable-voltage variable-frequency power source is used as power source for the re-circulation pump and controls both the power-source voltage and the power-source frequency.
The conventional variable-voltage variable-frequency power source for re-circulation pump has a semiconductor electric power converter and a speed controller. The semiconductor electric power converter has a semiconductor electric power forward converter and a semiconductor electric power inverter. The forward converter receives the three-phase alternating current (AC) from an in-plant electric power-supply system and converts the alternating current to a direct current (DC). The inverter converts the direct current to an alternating current of any desired frequency.
In accordance with the speed demand signal from the re-circulation flow controller, the semiconductor electric power inverter is controlled so that the ratio of its output voltage to its output frequency may be constant. The electric motor that drives the re-circulation pump is thereby controlled in speed. Ultimately, the speed of the re-circulation pump is controlled. The re-circulation pump and the electric motor operate in conjunction with each other.
In the variable-voltage variable-frequency power source, an over-current may flow in the semiconductor electric power converter if any semiconductor element constituting the converter fails to operate, assuming a short-circuiting state. When the speed controller detects a semiconductor-element failure signal that indicates the generation of the over-current, it stops supplying the semiconductor-element control signal, to the semiconductor electric power converter. The variable-voltage variable-frequency power source is thereby stopped. As a result, the re-circulation pump is stopped. This protects the other elements and the electric motor.
Inspection and maintenance work cannot be performed on the semiconductor electric power converter while a voltage is kept applied to the converter. Inevitably it is necessary to stop the variable-voltage variable-frequency power source. Hence, the re-circulation pump must be stopped in order to perform inspection and maintenance work on the semiconductor electric power converter.
The above-mentioned reactor-coolant re-circulation system has two sub-systems in most cases. Each sub-system has a re-circulation pump. One re-circulation pump may be stopped when any one of the semiconductor elements of the variable-voltage variable-frequency power source fails to operate or when the semiconductor electric power converter is undergoing inspection or maintenance work. In this case, the other re-circulation pump keeps operating. Hence, the coolant can be circulated in the reactor pressure vessel, though the coolant flow rate may decrease.
However, the coolant may stop flowing in the loop pipe provided in the sub-system in which the re-circulation pump is stopped. In this case, the coolant temperature in the loop pipe decreases. Consequently, the difference between the temperature in the loop pipe and that in the reactor pressure vessel increases. The increase in the temperature difference results in an increase in thermal fatigue at the junction between the reactor pressure vessel and the loop pipe. The increase of thermal fatigue adversely influences the plant lifetime.
In addition, when one of the re-circulation pumps is stopped, the coolant re-circulation flow rate may decrease in the core of the reactor. If this is the case, the output power of the reactor will decrease.
Therefore, it is desired that the re-circulation pump is not stopped. However, in the conventional variable-voltage variable-frequency power source for re-circulation pump, the re-circulation pump is stopped when any one of the semiconductor elements of the semiconductor electric power converter fails to operate or when the semiconductor electric power converter is undergoing inspection or maintenance work.
U.S. Pat. No. 5,625,545 discloses a technique of preventing a re-circulation pump from stopping even if one of the semiconductor elements fails to operate, the disclosure of which is hereby incorporated by reference in its entirety. In this technique, semiconductor electric power inverters are connected in series, forming a redundancy inverter unit, one bypass circuit is provided for each semiconductor electric power inverter, and a switch is provided on each bypass circuit. If one of the semiconductor elements fails, the semiconductor electric power inverter including this element is switched to the bypass circuit. The power source can therefore keep operating. In the technique disclosed in U.S. Pat. No. 5,625,545, however, the variable-voltage variable-frequency power source for the re-circulation pump must be stopped in order to repair the failed element. Consequently, the re-circulation pump is stopped.